One important part of many analog and digital devices is a generator for a voltage reference or a current reference which exhibits good characteristics. Here, “good characteristics” means that the generator for the reference generates a voltage or a current that is stable or changes only nominally over a wide range of supply voltages (which typify battery-powered devices) and over a wide range of temperature variations, and which can also be implemented with existing fabrication processes, preferably as part of other circuitry for the device.
The bandgap reference is a popular reference generator that successfully satisfies these requirements. Its principal of operation is widely understood: essentially, the bandgap reference uses cancellation from components that exhibit proportional-to-absolute-temperature (PTAT) characteristics and components that exhibit complementary-to-absolute-temperature (CTAT) characteristics, so as to generate a voltage that is relatively independent of temperature and supply voltage.
One drawback of conventional bandgap reference circuitry is that, when supplying a reference voltage, the lowest possible reference voltage is limited to a low value of approximately 1.27v. With increasing emphasis on reductions in power, so as to extend battery life, reduce power consumption, and reduce heat generation, the industry has worked to develop a bandgap reference circuit that provides a voltage reference whose low-level voltage is less than 1.27v.
For example, Banba, et al. introduced a bandgap reference circuit with sub-1v operational output. See Banba, et al., “A CMOS Bandgap Reference Circuit With Sub-1-v Operation”, IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, p. 670-674 (May, 1999). The principle of operation for such a bandgap reference circuit will be explained with respect to FIG. 1.
FIG. 1 shows a bandgap reference circuit 10 according to the proposal of Banba, et al. The bandgap reference circuit 10 includes a bandgap core 11 and an output circuit 12 for outputting a reference voltage Vout. Bandgap core 11 includes a pair of complementary first and second diode/resistor networks that exhibit PTAT and CTAT characteristics, respectively. The PTAT diode/resistor network comprises resistor R2A connected in parallel to series-connected resistor R1 and N diodes D1. The CTAT diode/resistor network comprises resistor R2B connected in parallel to a single diode D2. Typically, resistors R2A and R2B have the same value, i.e., R2A=R2B. First and second current mirrors comprising transistors MPA and MPB are driven from a common PMOS bus 14, and feed current to the first and second diode/resistor networks. Because the currents are mirrored, current iA through MPA is equal to current iB through MPB. A differential amplifier OP1 has its differential inputs driven by voltage VA from the PTAT diode/resistor networks, and by voltage VB from the CTAT diode/resistor network. The output of OP1 drives the common PMOS bus.
Output circuit 12 outputs the voltage reference Vout, and comprises a third current mirror which includes transistor MP3 driven from the common PMOS bus together with a series-connected resistor R3. Since transistor MP3 is driven from the same PMOS bus 14, current i3 through transistor MP3 is the same as currents iA and iB. The output circuit acts to combine (such as through addition and multiplication) voltages produced by the PTAT and CTAT diode/resistor networks, thereby producing a reference voltage Vout that is stable over a wide range of temperatures and supply voltages VDD. In addition, the generated reference voltage is lowered relative to pre-1999 bandgap references by the ratio of resistor R3 to resistor R2A, thereby achieving sub-1v operation. Furthermore, because of its implementation using CMOS components, fabrication of the FIG. 1 bandgap reference circuit can be achieved on a shared basis of other components of analog and digital devices.
FIG. 1 shows that the output of a bandgap reference is not necessarily limited to the output of a reference voltage. A reference current can also (or alternatively) be provided. As shown in FIG. 1, transistor MP3′ is driven from PMOS bus 14, and outputs a current reference iout. In the case that transistor MP3′ is identical to transistor MP3, the value of iout is iout=Vout/R3.
One problem with the FIG. 1 is evident: its start-up characteristics are unpredictable and can be unstable. The Banba, et al. authors pointed out that a stable, but unusable, state of the circuit can be obtained where VA=VB=0. To avoid this undesirable state, Banba, et al. proposed a current injection device indicated generally at 15 directly onto PMOS bus 14, in the form of a power-on reset (POR). Such an approach has its own difficulties. In 2002, a researcher named Andrea Boni explained that current for the POR injection might or might not be stable or even available, since the device was itself in the process of power on. See Boni, “Op-Amps and Start-up Circuits for CMOS Bandgap References with Near 1-V Supply”, IEEE Journal of Solid State Circuits, Vol. 37, No. 10, p. 1339-1343 (October, 2002). Boni proposed direct injection of currents at 16, in the form of current IX and IY, so as to perturb the diode/resistor networks and force the diodes into a current-conducting state.
The inventor herein has observed that even with the aforementioned provisions for start-up, there are still situations where the bandgap circuit of FIG. 1 can fail to start or can start to an unreliable and unstable operational state. Consider, for example, a situation where VA=VB=0.6 volts (approximately), which is substantially smaller than the forward bias diode voltage of 0.7 volts. In this situation, there is zero current flowing through the diodes D1 and D2 of the diode/resistor networks, which signifies a failed start-up of the bandgap reference. However, because current can flow through resistors R2A and R2B, the bandgap circuit can still achieve stable operation that is quite close to a true bandgap solution (i.e., within around 80%), thereby making it impossible to detect such undesirable operational points simply by sampling VA or VB. This undesirable operating point is different from that noted by Banba, et al., which occurs at VA=VB=0. As a consequence, the system is weakly unstable, and might oscillate for a long period of time before reaching a desired operational point at which there is current flow through diodes D1 and D2.